The present invention relates to a method of inspecting a photomask reticle used in the fabrication of a semiconductor device. More specifically, this invention relates to a method of inspecting a photomask reticle for a large scale integrated circuit (LSI) or a very large scale integrated circuit (VLSI), using inspection data stored in a storage medium such as a magnetic tape. This method is referred to as a database inspection method.
Recently, as a result of the tremendously high packing density of semiconductor devices, the photomask pattern is becoming dazzlingly complex and microscopic. Since the photomask reticle corresponds to a mother pattern for fabricating the semiconductor device, it is most important to keep the reticle in excellent condition. A defect in its pattern, even a very microscopic one, cannot be allowed to exist.
Typically, in a LSI or VLSI semiconductor die, a plurality of semiconductor elements having the same pattern are fabricated. The inspection of the photomask reticle for this purpose was at first performed visually by the human eye using a microscope for comparison with an adjacent identical standard pattern. This visual inspection was then replaced by a "pattern comparing inspection" (also referred to as a "mask comparing inspection"), namely an automated optical comparison inspection using an image sensor such as a charge-coupled device (CCD) camera and an electronic circuit for comparing the image with a standard pattern.
However, recently, the pattern of the die has become so complicated that it becomes difficult to inspect correctly by only the pattern comparing inspection method. Further, as the pattern comparing inspection requires a standard pattern for the comparison, if the standard pattern has a defect, the whole inspection is caused to be a failure. Furthermore, if the die is composed of only a single pattern which does not have portions with the same shape and size, the pattern comparing inspection cannot be applied. On the other hand, a method of designing the reticle pattern by a computer technique, such as a CAD (computer aided design) system, using a "design database" has been developed. Consequently, the concept of the CAD system has been introduced to the inspection process, so that the inspection has come to be made by an "inspection database", which inspection is referred to as a "database inspection".
When the database inspection was first introduced, the design data was used for the database inspection, but lately an "inspection database" is itself provided, because it makes the inspection process easy and avoids any overlapping faults as between both the fabrication and the inspection. By applying the database inspection method, the accuracy of the inspection becomes very high.
The database inspection can be made by an apparatus called a "reticle tester", in which the inspection data for the reticle pattern is stored in a storage medium such as a magnetic tape. However, the volume of the inspection data becomes enormous because of too much complexity and too many kinds of the recent semiconductor devices, so that providing a space to stock the storage media has become almost impossible, and the inspection has come to require a large amount of time. These are becoming big problems for inspection of photomask reticles, but the database inspection method is needed to maintain the very high accuracy.